This invention relates to a representing utensil for use in making an invisibly depicted matter visible again, more particularly, a utensil which is designed so as to enable the depicted matter, which has been depicted with a coloring agent including an alkaline pH-indicator, to repeatedly develop color by the use of a coloring assistant including an alkaline pH-adjusting agent. Heretofore, there have been proposed various kinds of representing utensils by means of which depicted or printed matters being produced in an invisible state are to be made to develop color later on. By way of example, as seen in Japanese Patent Application Disclosure No. 135720 of 1976, there is a system on which an invisibly printed matter produced with an ink containing such a coloring agent as metallic salts of n-alkyl piridium is made to develop color by using a color-manifesting liquid containing salicylic acid. However, in such a system using the color-manifesting liquid of this kind, when the coloring has been done even once, it became impossible to repeat. On the other hand, as seen in Japanese Utility Model Publication No. 52063-1981, there is proposed a set for printed matters wherein the color is developed by causing a coloring assistant containing sodium carbonate and zinc chloride to act on its invisible printed part wherein the printed part is produced with a printing ink containing phenolphthalein. However, in the matter thus produced, the coloring is not only unclear, rather it is intended that the coloring assistant perpetuate the coloring as long as possible after the invisible printed part once has developed color by making its coloring agent contain zinc chloride and the like for the purpose of preventing the color-developed matter from growing weak with the passage of time and fading away in the end.
As mentioned above, there has not been heretofore proposed in any of the prior art a device which is intended to develop color, and then, of its own nature to lose its color in a short time after having developed color and to be reused by repeating the coloring and fading cycle.
On the contrary, our invention has for its object the provision of a representing utensil which has the ability to make visible whenever occasion calls for a depicted matter being invisibly prepared by handwriting or by stamping while using a coloring agent containing a pH-indicator, and further to make it possible not only to obtain fading color in a relatively short time but also to permit the repeated performance of the process of coloring and fading a number of times.